brachiopods and bivalves · the term ‘brachiopoda’ comes from ancient greek, the prefix...

CARLETON UNIVERSITY

Brachiopods and Bivalves

A theory on the morphology

ERTH2312A

Paleontology

Presented to Professor T. Patterson

Joachim de Fourestier

Student Number: 101022736

27/03/2017

Brachiopods and Bivalves are superficially similar, but are rather distantly related. Both

are twin-valved and are filter-feeders. That said, what caused these morphological

convergences or possibly divergences? What similarities are there? What distinguishes

them from one to another?

Joachim de Fourestier Winter 2017

SN#101022736 Brachiopoda and Bivalvia ERTH2312A

Earth Sciences, Carleton University Page 1

Abstract

The Brachiopoda phylum is compared to the Bivalvia class. An attempt is made at

explaining what could have influenced their morphology as well as to explore what

similarities and differences these two groups exhibit. They are commonly confused due

to their similar external shells. Both of these are organisms with “valves” (somewhat

symmetrical), but many differences that exist between the two: these might not seem

quite as obvious at first sight. The most common way of differentiating them is using the

symmetry content these two groups expose and express differently. Brachiopoda have

their plane of symmetry perpendicular to the hinge of the valves. In contrast, bivalvia

(with exceptions from scallops and oysters) have a plane of symmetry parallel to this

hinge. The morphological changes are assumed to be the direct result of natural selection.

However, organisms are known to able to change morphologically during their lifespan.

It has been concluded that the superficial resemblance is from the same ancestors rather

than a result of convergent evolution. Both of them are found to occupy similar niches

but exhibit a few differences. Brachiopods mainly focus on simply keeping a single

location that is stable and have developed many different apparatus in order to achieve

this goal. Bivalves are more flexible in that are mobile. Both are able to burrow down

into the sediment, but bivalves are able to go much deeper or change their lateral location

altogether if necessary. Knowing the significant advantages that bivalves have over

brachiopods and that they can share similar lifestyles, it is somewhat surprising that both

still coexist to this day. This is most likely due to the slightly different environments that

these two can occupy.




Introduction

Brachiopoda and Bivalvia are often confused by their similar exterior, but have some

differences in live styles and internal anatomy. Although the two are not closely related,

their morphology of a twin-valved shell is possibly due to similarities in their ecological

niches. That being said, it is most likely that they originated in a similar niche and

became divergent more internally then externally as they evolved with time. They have

can have similar lifestyles but have come to different adaptations to arising external or

environmental pressures such as salinity, competition, temperature changes, substrate

type, etc.

Brachiopod Morphology

The term ‘Brachiopoda’ comes from Ancient Greek, the prefix brachio– meaning

something that is related to an arm and the suffix –pod meaning foot. Brachiopods,

commonly known as lampshells, are amongst the most successful invertebrate phyla.

Their first appearance datum is within the Early Cambrian which collides with the well-

known and so-called “Cambrian Life Explosion”. They have ever since evolved,

diversified and lived on throughout the Paleozoic dominating low-level benthic

suspension feeding. They are mainly composed of a pedicle (resembling an “arm” or

“lamp stand”) and a shell with two valves (resembling a “foot” or “lamp”). From the

outside, they might seem almost the same as bivalves. However, their feeding methods,

life styles and internal structures (such as soft tissues) are quite definitely different from

one another (see Figure 1).

Brachiopods are sessile (non-mobile): most remain permanently attached to the

substrate via their external stalk-like appendage known as the pedicle: it essentially

allows them to maintain or “anchor” their position in the water column. As with most

suspension feeders, they tend to remain in a local area and extract food from the water

around them. They are able to do this using a structure known as a lophophore: this organ

can be used for both feeding and respiration. These are mainly rings or crowns with fine

tentacles. These tentacles are covered with fine little “hairs” known as cilia. These are

also responsible for generating the “feeding current”: a current of water with food

particles that flows towards the mouth or the gape. That being said, there is mucus found

on these cilia which essentially enable them to create adhesion with the food particles to

filter them out from the indrawn water. The lophophore is supported by the brachium.

This can be a calcareous structure in some species, otherwise known as a brachidium.

Common types of this support structure include brachiophores (a pair of prong-like

extensions), spiralia (a pair of spiral-like or coiled structures) and “loops” (hoop-like

structures). The mentioned examples are all attached to the brachial valve (described

later).




The two valves of the shell are different: one ventral and one dorsal. One of the

valves is termed the pedicle valve where the stalk-like appendage is attached: this can be

the ventral or dorsal valve depending on the species. Dorsal and ventral are terms used in

relation to the internal morphological features (soft-body parts) of the organism. Bearing

this in mind, brachiopods generally have their pedicle exiting from the ventral valve. The

currently living Magellania is an example of this. The other valve is referred to as the

brachial valve where it contains supports for the lophophores. Following with our last

example, this would be the upper or dorsal valve. Evidently, the pedicle valve is larger

than the brachial valve due to the pedicle opening. Some extinct brachiopods have

evolved and lost their pedicles: a free-living mode. However, they were still benthic,

lying idle on or partially buried in the seafloor. Even though brachiopods tend to have

shells with different size valves, all of them exhibit bilateral symmetry. The plane of

symmetry is perpendicular to the hinge line. In other words, this plane cuts

perpendicularly through both valves of the shell.

The umbo or beak is the initial point where the valve starts growing (or secreting).

Valves with radial ornamentation generally point towards this origin. Opposite to the

posterior or apex (where the beak and hinge is located) is the opening of the valves: this

is the anterior. The margin or trace between this “opening” is termed the commissure. In

one species this can be a relatively straight feature whereas in another, this can be

sinuous. For example, Waconella wacoensis has a relatively linear commissure whereas

the Zygospira modesta has a zig-zag trace. Most brachiopod shells today have a mineral

composition of calcium phosphate and chitin (a complex, long-chain polymer). This is

usually easily recognisable by its enamel-like lustre. Others have calcitic or calcium

carbonate shells like many marine organisms.

Brachiopods use a system of muscles to open and close their valves. As with most

fossils, soft tissue parts such as these muscles are not commonly preserved. However, the

impressions or scars are usually still visible. The adductor scars are where the closing

muscles were attached. Its opposite, the diductor scars, is where the opening muscles

were attached.

Some brachiopods exhibit dentition and were archaically grouped under two

classes: Inacticulata (no teeth or sockets) and Articulata (those with both teeth and

sockets). These teeth are knob-like protrusion located on the hinge of the pedicle valve.

Sockets are small depressions on the hinge of the brachial valve. The teeth fit into the

sockets to assure the appropriate opening and closing of the valves. That being said,

brachiopod dentition is nowhere near as complex as bivalvian dentition (discussed later).




Brachiopoda - Life Environments and Modes

Naturally, these bottom dwellers are all exclusive to a marine environment.

However, various species can inhabit different depths and regions of the ocean.

Brachiopods are typically oriented vertically to the substrate during their life time. That

said, they can be inclined and even horizontal. Additionally, vertically oriented ones tend

to have shells with valves that are equally biconvex whereas inclined or horizontal ones

will have unequal valve shapes such as concavo-convex or plano-convex. Brachiopods

vary greatly in shapes and sizes from a few microns to tens of centimetres in length.

Considering their general morphology, they have not changed all that much compared to

other phyla such as Chordata or even Mollusca. There are only about 120 genera of them.

These bottom dwellers have always essentially been twin-valved filter-feeding shellfish,

opening and closing their shells, waving around lophophores. Despite the tough

competition from the Bivalvia, brachiopods are still around. They live in many different

conditions from near-shore or intertidal environments to very deep basins over 6

kilometres below sea level. The larger species tend to live in these deeper environments.

Note that the larger size is more likely to be the consequence rather than the cause of

living in such an environment. Possible reasons could be due rarer competition (as not

many organisms can tolerate such depths or pressures), relatively more (or rather less

divided) food supply, most particles fall and land on the seafloor.

Brachiopod life styles can be classified based on its relation with the substrate.

When the animal lives completely buried within the seafloor, it is known as Infaunal.

Those that do live this way commonly have their posterior oriented downward and can

stabilize themselves by projecting their pedicle further downwards. For those that are

only partially buried are known as Semi Infaunal and are not necessarily attached to the

substrate by their pedicle. Those that are Epifaunal essentially live on or above the

substrate rather than in it. They are generally attached to the seafloor or other objects

(such as marine plants) by their pedicle (discussed later). Reclining or free-living

brachiopods (essentially an unattached lifestyle) are those that are horizontally floating

on (or partial in) the substrate (not to be confused with epifaunal). These will generally

have a plano-convex or concavo-convex shell shape. Some of them are modified to have

spines or larger surface area to help float atop the sediment. Additionally, some of these

have no pedicle opening, but will expose attachment point for those with external spines.

Although brachiopods are sessile, they have developed many different adaptations

(see Figure 2 for following morphologies). Some cement themselves to rocks or other

hard-shell animals such as Craniops or Schuchertella. Others such as Craniids or

Disciniids live together by encrusting hard surfaces (similarly to barnacles). Some species

have clasping spines can attach and cling on to other marine lifeforms and feed from

there instead of on the substrate acting as an epibiont such as Linoproductus and




Tenaspinus. The Chonetes is example of a reclining brachiopod that essentially supports

itself on top of the substrate by having its weight divided due to its large surface area

shell. A similar case is the Waagenoconcha, which accomplishes the same task using

many external spines. However, this morphology can be more advantageous since it does

not need the surface to be flat and can adjust its anterior at different angles relative to the

substrate. Finally, the “Mobile” brachiopods subdivide into the previously discussed

Infaunal and Semi-Infaunal lifestyles. Note that “mobile” is mentioned in the sense that

can move vertically but not necessarily laterally: their movement is limited to burrowing.

The Linguloids (resembling to Lingula) are commonly infaunal. Camerisma and

Magadina are examples of the semi-faunal type.

Brachiopods seem to be able to change their shell shape and life mode during

ontogeny (from birth to maturity). A study from the San Juan Islands, show the shells to

be more lopsided and asymmetric with increasing hydroenergy. (Schumann, 1991)1 This

strongly suggests that morphologic changes not only occur as a result of evolution but

also as a result of ecology.

Bivalve Morphology

The term ‘Bivalvia’ quite evidently comes from prefix bi- meaning two and valve,

and that is quite frankly what they look like. In contrast to Brachiopoda, Bivalvia is a

class, not a phylum. From a taxonomic and ecological standpoint, they are far more

diverse: a taxonomically lower rank expressing more heterogeneity. Like brachiopods,

bivalves (with exceptions from scallops and oysters) are also bilaterally symmetrical with

a twin-valve shelly exoskeleton (see Figure 3). Bivalves first appeared in the Early

Cambrian and started out as burrowers. Their diversity was originally fairly limited. It

was not until the Mesozoic that radiated becoming very successful burrowers and the

now second largest class within the Mollusca phylum. The section is slightly briefer as

many parts found in bivalves are analogous to those of brachiopods.

In contrast to brachiopods, bivalves do not have diductor muscles to open their

shells. Instead, they have a ligament that automatically opens the valves because of its

elasticity. This ligament can be internal like in oysters or mussels, but it may also be

external in others. To close the valves or rather to hold them shut, bivalves usually have

two adductor muscles: contracting one of them accomplishes this task. Most bivalves

have mirrored valves and are referred to as left and right valves instead. When this is the

case, it is said they are equivalved. Asymmetrically valved ones are said to inequivalved.

Similar to its distant cousins (Brachipoda), these organisms also have an umbo where the

first parts of the shell are secreted. The composition of the shell is biomineralized

calcium carbonate such as calcite, aragonite (in higher temperatures) or even vaterite.

1 From Introduction to Paleobiology and the Fossil Record




However, some bivalves produce nacre (discussed later). In addition, the valves are

attached together by a toothed hinge (somewhat analogous to interlocking door hinges).

This structure ensures the valves close correctly without misalignment. There are eight

different types of teeth that can be observed with bivalves: desmodont (reduced to

absent), dysodont (small, simple teeth near the edge of the valve), taxodont (multiple

teeth, subequal, subparallel), actinodont (multiple, radially fanning out towards the

umbo), isodont (large teeth located on both side of the ligament’s depression), schizodont

(large, diverging, sometimes grooved), pachyodont (rather large, blunt) and heterodont

(cardinal, very large, lateral).

In contrast to brachipods, bivalves have a muscular foot instead of a pedicle. This

structure is use for attachment to the substrate, or to further burrow down into the

seafloor. This foot generally protrudes from the opening of the shell (anterior). Bivalves

use siphons to feed which located near the pallial sinus (similar to gape in Brachiopoda).

One of them, the inhalant siphon, is to drawn in water towards the internal cavity. The

other, known as the exhalant siphon, brings the water back out and away from the

internal cavity. When the water is drawn in, it passes through ciliated gills. These gills are

essentially able to filter out food particles and oxygen. There are four main types (not

explained here): protobranch, filibranch, eulamellibranch, septibranch.

Bivalvia - Life Environments and Modes

As with brachiopods, Bivalvia can also be classified by its relations to the

substrate (see Figure 4). The Infaunal bivalves can be subdivided into Shallow Infaunal

and Deep Infaunal. The shallow type commonly lives in the substrate of a river or sea.

Most have a height to length close to 1:1. They typically have a smooth and streamlined

shell which renders burrowing a much simpler task. They may have spines to firmly

attach themselves to the substrate so that they are not so easily removed by predators. The

deep type will live submerged with the substrate and their length is usually twice its

height. Burial is achieved by opening the valves and pushing the muscular foot

downwards into the sediment. This foot can also be used for locomotion. This is however

a relatively much younger lifestyle dating back to roughly 2 million years ago. Since

living and burrowing at such depths is difficult, it took longer to evolve fused siphons

(inhalant and exhalent) to accommodate this mode.

There are also Epifaunal bivalves. Instead of a pedicle to hold their ground, they

have the ability (that all bivalves have) to secrete byssal threads. These are sticky threads

that are typical used during infancy for when additional stabilization is needed. That

being said, there are some that use this ability during adulthood. The Mytilus is example

of this. They are known as epibyssate bivalves because they use these threads (that reach

down in the substrate) for stability and maintain this life style. This is somewhat




analogous to spider webs. Semi Infaunal or Endobyssates are similar but are partially in

the sediment: they combine the substrate and these thread for even more stability. These

typically have a pointed anterior. Given these points, bivalves that employ byssal threads

during their adulthood are typically found in high current environments. Next, cementing

and reclining bivalves are commonly inequivalved. Just like reclining brachiopods,

reclining bivalves can also have spines to help with stabilization. Unique to bivalve is

swimming. This offers a huge advantage over brachiopods. Swimming bivalves are

usually equilateral but they are inequivalved. The lower valve is usually larger. They

typically exhibit a wider umbonal angle (greater to 105°) and have a single, large,

centrally located adductor muscle.

Although most bivalves have shells that are primarily composed of calcium

carbonate, some can produce a different material known as nacre (such as Pinctada also

known as the “pearl oyster”). Calcareous shells are hard, robust and provide generally

good protection. However, it is very susceptible to chemical weathering and is relative

brittle. Chitin (complex, long-chain polymer) is quite resilient and pliable (so not very

brittle). With this in mind, nacre has the advantage of both calcium carbonate and chitin.

This biomaterial (organic-inorganic) composite consist of multilayered structures of

calcium carbonate and an elastic biopolymer such as chitin. More precisely, it is

comprised of micro aragonite tablets that usually have a rectangular, hexagonal or

rounded shape. This combination gives it a tough, hard structure that will rather deform

that shatter. Nacre has even been used in the material science industry to produce tougher

materials. It is considered to be almost as resilient as silicon. An interesting fact is that

pearls are the result of an autoimmune reaction known as “Nacrezation”: when foreign

material such as an irritant (such as sand) or a parasite, the host will secrete nacre around

it. Sometimes it is incorporated and creates bump within the animal. Otherwise, the

irritant essentially become the nucleus of a pearl. The goal of this behaviour to isolate the

irritant or to the very least creates a smoother surface. This seems to be the next big step

in their evolution to have a greater tolerance towards both mechanical and chemical

weathering.




Conclusion

To summarize, lophopores are too complex and distinctive of a structure for it to

be implicated in convergent evolution. A twin-valved exoskeleton currently seems to

primarily involve the specific lifestyle of filter feeding. Species that have this

morphology are more likely to be from one common point of origin: Lophotrochozoa.

Both Bivalvia and Brachiopoda are members of this superphylum. However, Bivalvia use

siphons for feeding whereas Brachiopoda use lophophores. This is one of the most

important distinctions between the two. Both of them have a shell that can be composed

of calcium carbonate. However, brachiopods tend to have a more phosphoritic

composition (such as chitin which is less susceptible to chemical deterioration). And

bivalve shells are primarily calcareous. That being said, the shell can provide protection

against predators and desiccation. Both have adopted similar forms of burrowing

lifestyles and tend to remain near the substrate. For filter feeding, brachiopods use

lophophores whereas bivalves use siphons. Both started to appear in the early Cambrian:

bivalves already had toothed hinges, but articulate brachiopods only started to appear in

the late Cambrian. Bivalves seem to have taken this evolution step much sooner. The

Jurassic involved losses of both brachiopods and bivalves, but free swimming animals

were not affected. This is most likely due to other lifeforms starting to also take

advantage of the substrate disturbing pre-existing life. It is possible that many of the

brachiopods were tipped over and bivalves were completely submerged by “sediment-

bulldozing” organisms, ultimately leading to the extinction of some species. Bivalves can

live in anywhere from freshwater to brackish waters. With this in mind, they are

commonly found in the shallower subtidal environments whereas brachiopods will prefer

deeper, calmer ocean waters. Both exhibit similar external morphology but their internal

anatomies are quite different. Brachiopods have developed many different complex

structures to achieve stability such as clasping spines and various shell shapes. In

contrast, bivalves have adopted different life modes such as swimming or moving to

different areas using their muscular foot. The concluded distinction is brachiopods

changed morphologically to maintain a lifestyle whereas bivalves changed

morphologically to adopt new lifestyles. That said, both are well adapted to their

environments and have been generally very successful.




References

- “Brachipoda”, Michael J. Benton and David A.T. Harper, “Introduction to

Paleobiology and the Fossil Record”, 2009, p. 298-309

- “Inarticulata: Characters and Anatomy”, University of Bristol, Earth Sciences,

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Inarticulate/Characters%20an

d%20Anatomy.html , retrieved March 2017

- “The Lophophore”, University of California - Berkeley, Earth Sciences,

http://www.ucmp.berkeley.edu/glossary/gloss7/lophophore.html ,

retrieved March 2017

- “Brachiopoda: Morphology and Ecology”, State University of New York,

Cortland, Paleontological Laboratory,

http://paleo.cortland.edu/tutorial/Brachiopods/brachmorph.htm ,


- “Bivalvia”, State University of New York, Cortland, Paleontological Laboratory,

http://paleo.cortland.edu/tutorial/Bivalves/bivalvia.htm , retrieved March 2017

- “Bivalvia: Morphology”, State University of New York, Cortland,

Paleontological Laboratory,

http://paleo.cortland.edu/tutorial/Bivalves/bivalvemorph.htm ,


- “Bivalvia”, Michael J. Benton and David A.T. Harper, “Introduction to

Paleobiology and the Fossil Record”, 2009, p. 326-338

“Introduction to the Lophotrochozoa”, University of California - Berkeley, Earth

Sciences, http://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html ,


- “Bivalvia: Modes of Life”, University of Bristol, Earth Sciences,

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Bivalvia/Modes.html ,


- “Bivalvia: Characters and Morphology”, University of Bristol, Earth Sciences,

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Bivalvia/Characters.html,


- “Mollusk shell formation: Mapping the distribution of organic matrix components

underlying a single aragonitic tablet in nacre”, Fabio Nudelman, Bat Ami Gotliv,

Lia Addadi, Steve Weiner, Journal of Structural Biology vol.153, 2006, p. 176–

187

- “Evolution of Immune Reactions”, Petr Sima, Vaclav Vetvicka, CRC Press, 1990,

p. 85

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Inarticulate/Characters%20and%20Anatomy.html

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Inarticulate/Characters%20and%20Anatomy.html

http://www.ucmp.berkeley.edu/glossary/gloss7/lophophore.html

http://paleo.cortland.edu/tutorial/Brachiopods/brachmorph.htm

http://paleo.cortland.edu/tutorial/Bivalves/bivalvia.htm

http://paleo.cortland.edu/tutorial/Bivalves/bivalvemorph.htm

http://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Bivalvia/Modes.html

http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Bivalvia/Characters.html




Appendix

Figure 1

Retrieved (March 2017) from: “Brachiopoda: Morphology and Ecology”, State

University of New York, Cortland, Paleontological Laboratory,






Figure 2

Retrieved (March 2017) from:“Brachipoda”, Michael J. Benton and David A.T. Harper,

“Introduction to Paleobiology and the Fossil Record”, 2009,

http://www.blackwellpublishing.com/paleobiology/figure.asp?chap=12&fig=Fig12-

9&img=c12f009

http://www.blackwellpublishing.com/paleobiology/figure.asp?chap=12&fig=Fig12-9&img=c12f009





Figure 3

Retrieved (March 2017) from: “Bivalvia”, Michael J. Benton and David A.T. Harper,



5&img=c13f005






Figure 4

Retrieved (March 2017) from: “Bivalvia”, Michael J. Benton and David A.T. Harper,



9&img=c13f009



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